If you need help to estimate the growth rates, use this decision calculus applet. The applet shows you the growth dependig on the growth rate and the minimum and maximum input and ouput values you inidcate.
For example if you want to estimate how the margin evolves during the given number of periods, you need to fill the data mentioned below.
The minimum and maximum Inputs are the beginning and the end of the period. In this example they should be set between 0 and 15.
The minimum and maximum Outputs refere in this example to the margin.
For the sake of visual clarity use integer values. For the margin these values can be between 35 and 40 (meaning percent).
The reactivity is the growth rate. It usually should be smaller than one. Try several growth rates until you find the one that suites your case best.